tag: bound picture-data allocation in readPictureBlock - #115
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readPictureBlock read a 4-byte picture-data length straight from the file and then did make([]byte, dataLen) before reading, bypassing the readBytesMaxUpfront cap that the rest of the package uses. A malformed FLAC or Ogg/Vorbis file (or a metadata_block_picture comment) only a few dozen bytes long that advertises a multi-gigabyte picture therefore forces a correspondingly huge allocation, a memory-amplification DoS for anything running ReadFrom on untrusted input. Read the picture data through readBytes, which caps the up-front allocation and streams larger payloads. Valid files are unaffected.
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While reading a FLAC/Vorbis
PICTUREblock,readPictureBlocktakes the 4-byte picture-data length directly from the file and then doesdata := make([]byte, dataLen)before reading any of it. That path bypasses thereadBytesMaxUpfront(10 MB) cap that the rest of the package already uses viareadBytes, so an attacker-controlled length is trusted verbatim.Failure mode: a malformed FLAC or Ogg/Vorbis file (or a
metadata_block_picturecomment) only a few dozen bytes long can advertise a multi-gigabyte picture and forceReadFrominto a correspondingly huge allocation — a memory-amplification DoS for any caller reading untrusted files.Repro (before this change): a 40-byte
fLaCfile whose single metadata block is a PICTURE block declaring a 100 MiB data length makesReadFromallocate 100 MiB (and up to ~4 GiB with the maximum 32-bit length), even though no picture data follows.Fix: read the picture data through
readBytes, the same helper used elsewhere, which caps the up-front allocation and streams larger payloads instead of trusting the declared size. Valid files are unaffected.Tests: added
TestFLACPictureAllocationBounded, which decodes a tiny crafted FLAC and asserts the decode stays under 20 MiB (fails on the old code at 100 MiB, passes now). The existing test suite (which parses the real FLAC/Ogg fixtures with picture blocks) still passes;gofmtandgo vetare clean.